1. Field of the Invention
The present invention relates to a solid-state image pickup element and a driving method thereof, and a camera system, and more specifically to a solid-state image pickup element capable of capturing both still images and dynamic picture images and a driving method of the same as well as to a camera system using the same.
2. Description of the Related Art
A digital still camera (DSC) uses, as its image pickup device, a solid-state image pickup element, such as a charge coupled device (CCD), implementing so-called all-pixel-read-out mode, in which the signal charges of all pixels are simultaneously read out to a vertical transfer section, and are transferred and outputted individually without being mixed in the vertical transfer section. For those CCD image pickup devices intended for the use in DSCs, efforts have been made to increase the number of pixels in an attempt to improve the capability of capturing still images in better quality.
In a case of a digital still camera, means must be provided to allow monitoring an image being captured in order to adjust the focus or the angle of the camera during picture taking. Accordingly, a digital still camera is generally equipped with a monitor to display a captured image, such as a liquid crystal display. Also, it is designed to be capable of selectively implementing a monitoring mode besides the still image mode for capturing a still image. In order to be able to display an image captured by, especially, a high-pixel-count CCD image pickup element on the liquid crystal display while in this monitoring mode, it is necessary to increase the frame rate.
In a digital still camera using, as its image pickup device, a high-pixel-count CCD image pickup element implementing the all-pixel-read-out mode, a thinning technique has hitherto been used as one approach for increasing the frame rate in the monitoring mode. This thinning technique, as implemented in a CCD image pickup element, provides thinned pixel information by reading out only the signal charges of pixels in a subset of lines (rows) to the vertical transfer section, and leaving the rest of the lines unread. This thinning technique provides a reduction in the amount of data in the vertical direction, so that the frame rate can be increased.
When attention is directed to those pixels in the lines (rows) to be thinned out in such a CCD image pickup element capable of implementing the thinning operation, while these pixels are subject to the thinning during the monitoring mode, they are read out in no different manner as those pixels of other lines during the normal still image mode, so that the driving pattern of those rows to be thinned out is different in the monitoring mode and in the still image mode.
Accordingly, in a conventional CCD image pickup element capable of implementing the thinning operation, two driving systems (driving pulse, driving terminals, wirings, etc.) for the regular lines from which the signal charges are read out, and for those lines from which the signal charges are not read out (rows to be thinned out) are necessary, and in addition, the driving systems once configured cannot be modified later as they are hardwired, so that this approach can provide only a predetermined thinning rate. In other words, one cannot arbitrarily select the vertical compressibility.
In addition, the conventional thinning technique involves the thinning operation of pixel information performed on a row-by-row basis at the point where signal charges are read out from the respective pixels, so that it may provide pixel information thinned in the vertical direction, but not in the horizontal direction. This means that, in the horizontal direction, the signal charges of all the pixels would be read out. Accordingly, when the pixel count is increased as a result of the efforts to enhance the performance of high-pixel-count CCD image pickup elements, the horizontal transfer section would have to be driven at a higher rate, and resulting in an increased driving frequency, thus, increased power consumption.
Moreover, in a case where the thinning operation is implemented at the point when the signal charges are read out from the sensor section as explained above, smears and dark currents are generated also in those thinned out transfer stages of the vertical transfer section in the same manner as the other transfer stages in which signal charges are present even though they have no signal charges so that these smears and dark currents are eventually added to the signal charges in the horizontal transfer section, and the proportion of smear and dark current components relative to the signal component per pixel is increased, resulting in deterioration in the image quality.